This invention relates to surgical fasteners and their associated applicators, and more particularly, surgically fastening material to tissue and their method of use.
In laparoscopic repair of hernias surgical fasteners have been used to attach repair mesh over the hernia defect so that bowel and other abdominal tissue are blocked from forming an external bulge that is typical of abdominal hernias. The role of the fasteners is to keep the mesh in proper position until tissue ingrowth is adequate to hold the mesh in place under various internal and external conditions. Adequate ingrowth usually takes place in 6-8 weeks. After that time the fasteners play no therapeutic role. Fixation anchors comprise a mesh fixation feature, or head, a mesh-tissue interface section, and a tissue-snaring feature that holds the anchor in place under force developed inside or outside the body.
At present, there are a variety of surgical devices and fasteners available for the surgeon to use in endoscopic and open procedures to attach the mesh patch to the inguinal floor or abdominal wall. One such mesh attachment instrument uses a helical wire fastener formed in the shape of a helical compression spring. Multiple helical wire fasteners are stored serially within the 5 mm shaft, and are screwed or rotated into the mesh and the overlaid tissue to form the anchor for the prosthesis. A load spring is used to bias or feed the plurality of helical fasteners distally within the shaft. A protrusion extends into the shaft, while preventing the ejection of the stack of fasteners by the load spring, allows passage of the rotating fastener. U.S. Pat. Nos. 5,582,616 and 5,810,882 by Lee Bolduc, and U.S. Pat. No. 5,830,221 by Jeffrey Stein describe instruments and fasteners of this type.
U.S. Pat. Nos. 5,203,864 and 5,290,297 by Phillips describe two embodiments of a hernia fastener and delivery devices. One of the Phillips fasteners is formed in the shape of a unidirectional dart with flexible anchor members. The dart is forced through the mesh and into tissue by a drive rod urged distally by the surgeon's thumb. The anchor members are forced inward until the distal end of the dart penetrates the overlaid tissue and then the anchor members, presumably, expand outward without any proximal force on the dart thus forming an anchor arrangement. This requires an extremely forceful spring force generated by the anchor members. Multiple darts are stored in a rotating cylinder, much like a revolver handgun.
Phillips second fastener embodiment is a flexible H shaped device. The tissue penetrating means is a hollow needle containing one of the legs of the H. The H shape is flattened with the cross member and the other leg remaining outside the hollow needle owing to a longitudinal slot therein. A drive rod urged distally by the surgeon's thumb again delivers the fastener. The contained leg of the H penetrates the mesh and tissue. After ejection the fastener presumably returns to the equilibrium H shape with one leg below the tissue and one leg in contact with the mesh with the cross member penetrating the mesh and the tissue, similar to some plastic clothing tag attachments. Phillips depicts the installed device returning to the H shape but he fails to teach how to generate enough spring action from the device to overcome the high radial forces generated by the tissue.
A series of patents, U.S. Pat. Nos. 6,572,626, 6,551,333, 6,447,524, and 6,425,900 and patent applications 200200877170 and 20020068947 by Kuhns and Kodel, all assigned to Ethicon, describe super elastic, or shape metal fasteners and a delivery mechanism for them. The fasteners are stored in the delivery device in a smaller state and upon insertion into the mesh and tissue, transitions to a larger anchor shaped state. The Ethicon fastener is delivered by an elaborate multistage mechanism through a hollow needle that has penetrated the mesh and the tissue. The hollow needle is then retracted to leave the fastener to change shape to a more suitable configuration for holding the mesh in place.
The primary problem with these prior art fasteners is that the mesh is attached to body tissue in as many as 100 places for large ventral hernias. This results in a large quantity of metal remaining in the body as permanent implants, even though after the ingrowth phase the fasteners serve no useful purpose. Compounding this problem the distal ends of the fasteners are sharp pointed and thus pose a continued pain or nerve damage hazard.
One alternative to metallic fixation devices is bio-absorbable materials. These materials are degraded in the body by hydrolysis. After degradation the body metabolizes them as carbon dioxide and water. These materials require special attention to many design details that are much more demanding than their counterparts in metallic fixation devices such as applicator tool design, sterilization processes, and packaging. Metallic tacks or anchors provide structural strength that simplifies their insertion and since the materials, usually titanium or nickel-titanium alloys (shape metal), are chemical and radiation resistant and are very temperature tolerant many options are available to the designer. Not so for bio-absorbable materials.
The basic considerations of an effective mesh fixation applicator and absorbable anchor are the material strength, absorption time, the sterilization method, and packaging requirements, the ease of insertion of the anchor through the mesh and into the tissue, the ease of ejecting the anchor from the tool, the fixation strength of the anchor once implanted, the time required after insertion for the anchor to be degraded and metabolized by the body are all effected by the choice of anchor material, the geometry of the design, and the forming process.
Materials of appropriate strength are generally limited to synthetic materials. Currently, the U.S. FDA has cleared devices made from polyglycolide (PG), polylactide (PL), poly caprolactone, poly dioxanone, trimethylene carbonate, and some of their co-polymers for implant in the human body. These materials and their co-polymers exhibit a wide variation of properties. Flex modulus ranges from a few thousand to a few million PSI, tensile strength ranges from 1000 to 20,000 PSI, in vivo absorption times range from a few days to more than two years, glass transition temperatures range from 30-65 degrees centigrade, all with acceptable bio-responses. Unfortunately, however, the optimum values of each of these properties are not available in any one of these materials so that it is necessary to make performance tradeoffs.
Mechanical Properties
Most hernia mesh fixation devices are currently used in laparoscopic hernia repair. In general laparoscopic entry ports have been standardized to either 5 or 10 mm (nominal) diameter. In the case of prior art of metallic fixation devices 5 mm applicators are universally employed. Since it is not clear that the medical advantages of the use of absorbable anchors would totally out weigh the disadvantages of moving to a 10 mm applicator it must be assumed that absorbable anchors must also employ 5 mm applicators. Because of the lower strength of absorbable material this requirement imposes severe design constraints on both the applier and the anchor.
After successful insertion there are two ways for a fixation anchor to fail. It can fracture, separating the mesh holding feature from the tissue-snaring feature, or it can pull out of the tissue owing to inadequate tissue snaring. Increased forces are placed on the anchor during sudden elevations of intra-abdominal pressure (IAP) caused by straining, coughing or the Valsalva maneuver, a medical procedure whereby patients close their nose and mouth and forcibly exhale to test for certain heart conditions. The later can generate an IAP of up to 6.5 PSI. For nonporous mesh and a hernia area of 50 square centimeters, for example this increased IAP places 50.3 pounds of force on the anchors fixating the mesh. Typically 40 anchors would be used to secure the hernia mesh of 150 square centimeters so that each anchor would, at this elevated IAP, experience approximately a 1.26-pound tensile force on the mesh-retaining feature and the tissue-snare feature. The tensile strength between these two features and the tissue snare force must exceed this force generated by the increased IAP or else the mesh fixation can fail.
The strength and flexibility of the anchor material are of major importance in the design considerations of the applicator, particularly in the case of anchors formed from polymers. Ory, et al (U.S. Pat. No. 6,692,506) teaches the use of L Lactic Acid polymer. Ory discloses adequate fixation strengths but the applicator device required to insert his anchor is necessarily 10 mm in diameter thereby causing the procedure to be more invasive than necessary. Ory further discloses a hollow needle with a large outside diameter, through which the anchor is inserted, that forms a rather large hole in the mesh and tissue to supply adequate columnar strength for penetration of the anchor. Entry holes of this size can give rise to multiple small hernias know as Swiss cheese hernias.
Absorption Time
There are two forms of PL, one synthesized from the d optical isomer and the other from the l optical isomer. These are sometimes designated DPL and LPL. A polymer with 50-50 random mixture of L and D is herein designated DLPL.
High molecular weight homo and co-polymers of PG and PL exhibit absorption times ranging from 1 month to greater than 24 months. Homo crystalline PG and PL generally require greater than 6 months to absorb and thus are not optimum materials for hernia mesh fixation. Amorphous co-polymers of PG and PL, on the other hand, typically degrade in less than 6 months and are preferably used in the present invention. For high molecular weight co-polymers of PG and PL the actual absorption time is dependent on the molar ratio and the residual monomer content. For a given monomer residual the absorption time varies from about 1 month to about 5 months as the molar content of DLPL increases from 50 to 85 percent with PG decreasing from 50 to 15 percent. Co-polymers of DLPL and PG in the molar range of 50 to 85 percent of DLPL are preferred for this invention. The geometry of the anchor also effects the absorption time. Smaller high surface area devices absorb faster.
The time required for the human body to react to the foreign body of the mesh for tissue ingrowth into the mesh is typically 10 days. However, mesh migration and mesh contraction can occur for more than two months if not adequately stabilized. Since fixation anchors can impinge upon nerves and cause pain it is desirable for the anchors to be absorbed as soon as possible after the tissue ingrowth and after the mesh is secure against migration or contraction. For most absorbable materials there is a difference between the time for loss of fixation strength and mass loss. Fixation strength decreases quicker than anchor mass owing to some degree of crystalline structure in the polymer. For these reasons the preferred absorption time for the current invention is 3-5 months after implant.
Temperature Effects
Glass transition temperature (Tg) is the temperature above which a polymer becomes soft, can loose its shape, and upon re-cooling can shrink considerably. Both crystalline and amorphous polymers exhibit glass transitions in a temperature range that depends on the mobility of the molecules, which is effected by a number of factors such as molecular weight and the amount of residual monomers. Glass transition temperatures range from about 43 to 55 degrees centigrade (deg. C.) for co-polymers of PG and DLPL. Where as 100% PG has a Tg of 35-40 deg. C. and 100% PL exhibits a Tg from 50-60 deg. C. Since the core temperature of the body can reach 40 degrees C. the preferred Tg for the material comprising the current invention is greater than 40 deg. C. In addition hernia mesh anchors are often manufactured and shipped via surface transportation under uncontrolled, extreme heat conditions. Temperatures in commercial shipping compartments in the summer can exceed 60 degrees C. It is necessary then to provide thermal protection in the packaging so that the anchor temperature does not exceed its Tg.
Sterilization and Packaging
Bio-absorbable polymers degrade when exposed to high humidity and temperature. Autoclaving cannot be used, for example. Most ethylene oxide (ETO) sterilization processes employ steam and high temperatures (above Tg) to obtain reasonable “kill” times for the bio-burden commonly found on the device. High doses of gamma radiation or electron beam radiation (E Bream), both accepted methods of sterilization for many devices, could weaken the mechanical properties of PG, PL and their co-polymers. It is therefore necessary during the manufacturing process of the anchor and its applicator to maintain cleanliness to a high degree such that the bio-burden of the components is small enough so that pathogens are adequately eradicated with less severe forms of sterilization.
Radiation doses above 25 kilogray (kgy) are known to lessen the mechanical strength of bio-absorbable polymers whereas some pathogens are known to resist radiation doses below 10 kgy. It is therefore necessary, for the preferred embodiment of the present invention during manufacturing to keep the pathogen count below a certain threshold to insure the accepted regulatory standards are met for radiation levels between 10 and 25 kgy.
In a second embodiment of the present invention it is necessary during manufacturing to keep the pathogen count below a certain threshold to insure the accepted regulatory standards are obtained for sterilization using a non-steam, low temperature, ethylene oxide (ETO) process below Tg of the anchor polymer.
Anchors of the present invention must be carefully packaged to maintain adequate shelf life prior to use. Care must be taken to hermetically seal the device and to either vacuum pack, flood the package with a non-reactive dry gas prior to sealing, or to pack the device with a desiccant to absorb any water vapor since hydrolysis breaks down the backbone of the co-polymers.
ETO sterilization requires the gas to contact the device to be sterilized. Devices that are not humidity sensitive can be packaged in a breathable packaging material so that ETO can diffuse in, and after sterilization, diffuse out so that the device can be sterilized without unsealing the packaging. For the alternate embodiment of the present invention the device must be hermetically sealed after sterilization with ETO. Since gamma radiation and electron beam radiation sterilization can be accomplished through hermetically sealed packaging without disturbing the seal, either of these two sterilization processes is employed for the preferred embodiment of the present invention.
Ory, et al (U.S. Pat. No. 6,692,506), Criscuolo, et al (US application 20040092937), Phillips (U.S. Pat. Nos. 5,203,864 and 5,290,297), Kayan (U.S. application 20040204723), and Shipp (U.S. application Ser. Nos. 10/709,297 and 10/905,020) have suggested the use of bio-absorbable materials for use as hernia mesh fixation devices to solve the problems associated with the permanency of metal implants. Ory, preferably, suggests forming the fixation device from LPL but the absorption time for LPL can exceed two years, much longer than optimum for hernia fixation devices since the lessening of pain depends on mass loss of the device. While Phillips and Kayan advocate the use of bio-absorbable material to form the anchor neither teach any details or methods for effectuating such a device. Criscuolo suggests the use of PG and PL with an absorption time of 2-3 weeks but does not disclose a method of forming the device that results in such an absorption time. In any respect, migration and contraction of the mesh has been documented to occur up to 8 weeks after implant. Loss of fixation after 2 to 3 weeks could well lead to hernia recurrence.
Hernia mesh such as PTFE based mesh manufactured by W. L. Gore is difficult to penetrate since the material is tough, non macro-porous, and relative inelastic. Attempts to penetrate these types of meshes with a puncture type applicator result in the mesh indenting into the tissue to a significant depth prior to penetration, especially for soft tissue. This indentation sometimes allows the tissue penetrator means, often a hollow needle, to penetrate through the abdomen wall and into the surgeon's hand, thus exposing the surgeon to potential hepatitis and AIDS viruses. The anchor of the present invention is equipped with screw threads that easily penetrate tough, non macro-porous, and relative inelastic mesh with a minimum of indentation. Once the threads are screwed through the mesh the underlying tissue is pull toward the mesh by the threads rather than push away from the mesh as is the case with puncture type devices.
Details of the method of manufacturing the improved anchor are herein provided.
What is needed then is an absorbable mesh fixation anchor and a method of forming an absorbable mesh fixation anchor that exhibits a known absorption time and that exhibits the mechanical properties adequate for the desired fixation strength and the required implant forces.
What is also needed is a method of packaging an absorbable mesh fixation device and the delivery device that minimizes the effects of high ambient shipping temperatures and humidity.
What is also needed is a method of sterilization of an absorbable mesh fixation anchor and its delivery device that has minimal effect on their physical properties, particularly the anchor.
What is further needed then is an absorbable mesh fixation anchor of improved geometry that easily penetrates tough, non macro-porous, and relatively inelastic mesh with minimal indentation to minimize the possibility of the anchor breaching the abdominal wall.